I had a direct experience with these type of sites, but on the consumer sites. I found that most dominating providers are the ones from India, Nepal, Russia. Low cost, but a lot of times poor quality of work. They kill most bids with low prices and unsuspecting clients then have to hire local resources to either finish or correct some of the things they've rushed through.

That, of course, depends completely on the complexity and scale of the task. If I have something simple that needs to be done, I don't mind hiring anyone who can offer that at best price.